This invention relates to a signal processor for an apparatus that measures properties of a material flowing through at least one vibrating conduit in the apparatus. More particularly, this invention relates to a digital signal processor for performing calculations to determine the frequencies of signals receive from pick-off sensors measuring the frequency of vibrations of the conduit.
It is known to use Coriolis effect mass flowmeters to measure mass flow and other information for materials flowing through a conduit in the flowmeter. Exemplary Coriolis flowmeters are disclosed in U.S. Pat. Nos. 4,109,524 of Aug. 29, 1978, U.S. Pat. No. 4,491,025 of Jan. 1, 1985, and U.S. Pat. No. Re. 31,450 of Feb. 11, 1982, all to J. E. Smith et al. These flowmeters have one or more conduits of a straight or a curved configuration. Each conduit configuration in a Coriolis mass flowmeter has a set of natural vibration modes, which may be of a simple bending, torsional or coupled type. Each conduit is driven to oscillate at resonance in one of these natural modes. Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter, is directed through the conduit or conduits, and exits the flowmeter through the outlet side of the flowmeter. The natural vibration modes of the vibrating, material filled system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the flowmeter, all points along the conduit oscillate due to an applied driver force with identical phase or small initial fixed phase offset which can be corrected. As material begins to flow, Coriolis forces cause each point along the conduit to have a different phase. The phase on the inlet side of the conduit lags the driver, while the phase on the outlet side of the conduit leads the driver. Pick-off sensors are placed on the conduit(s) to produce sinusoidal signals representative of the motion of the conduit(s). Signals outputted from the pick-off sensors are processed to determine the phase difference between the pick-off sensors. The phase difference between two pick-off sensor signals is proportional to the mass flow rate of material through the conduit(s).
A Coriolis flowmeter has a transmitter which generates a drive signal to operate the driver and determines a mass flow rate and other properties of a material from signals received from the pick-off sensors. A conventional transmitter is made of analog circuitry which is designed to generate the drive signal and detect the signals from the pick-off sensors. Analog transmitters have been optimized over the years and have become relatively cheap to manufacture. It is therefore desirable to design Coriolis flowmeters that can use conventional transmitters.
It is a problem that conventional transmitters must work with signals in a narrow range of operating frequencies. This range of operating frequencies is typically between 20 Hz and 200 Hz. This limits the designers to this narrow range of operating frequencies. Furthermore, the narrow range of operating frequencies makes it impossible to use a conventional transmitter with some flowmeters, such as straight tube flowmeters, which operate in a higher frequency range of 300-800 Hz. Straight tube flowmeters operating at 300-800 Hz tend to exhibit smaller sensitivity to Coriolis effects used to measure mass flow rate. Therefore, a finer measurement of the phase difference between sensors is-needed to calculate mass flow rate.
In order to use one type of transmitter on several different designs of Coriolis flowmeters operating at several different frequencies, manufacturers of Coriolis flowmeter have found that it is desirable to use a digital signal processor to generate the drive signals and process the signals received from the pick-off sensors. A digital signal processor is desirable because the higher demand in measurement resolution and accuracy put on analog electronic components by flowmeters operating at higher frequencies, such as straight tube designs, are avoided by the digitalization of signals from the pick-offs as the signals are received by the transmitter. Furthermore, the instructions for signaling processes used a digital processor may be modified to operate on several different frequencies.
However, digital signal processors have several disadvantages as compared to conventional analog circuit transmitters. A first problem with a digital signal processor is that digital processors are more expensive to produce because the circuitry is more complex. Secondly, digital signal processors require a circuit board having a greater surface area which can cause problems when space is at a premium in a flowmeter design. Thirdly, digital signal processors require more power to operate than analog circuits. Power consumption is especially a problem when a processor must operate at a maximum clock rate in order to provide all the computations needed to process the signals and generate a material property measurement, such as mass flow. For all of these reasons, there is a need in the art for a digital signal processor that is adaptable across several flowmeter designs, that is inexpensive to produce and reduces the amount of power needed to perform the needed computations.
The above and other problems are solved and an advance in the art is made by the provision of a multi-rate digital signal processor of the present invention. The present invention is comprised of processes that are stored in a memory and executed by the processor in order to process the signals received from pick-offs on a vibrating conduit. The processes of this invention offer many advantages that make it viable to use a single type of digital signal processor in many types of Coriolis flowmeters.
A first advantage of the processes of the present invention is that the processes do not lose accuracy in spite of using finite arithmetic in lieu of floating point arithmetic. A second advantage of the processes of the present invention is that the processes can be implemented on any number of low cost, low power digital signal processors such the Texas Instruments TM3205xx, Analog Devices ADSP21xx, or Motorola 5306x. The instructions for the processes of the present invention are small enough to reside in the internal memory of a digital signal processor which eliminates the need for fast access external memory which increases the cost, power consumption and board space for the transmitter. The processes have a small number of computational constructs which improves the portability of the processes between low cost processors.
A third advantage is that the computational requirement of the processes is minimized. This reduction in the computational requirement allows the digital signal processor to run at a clock rate lower than the maximum clock rate of the processor which reduces the power consumption of the processor.
A transmitter that performs the processes of the present invention has the following electronic components. Analog signals from the pick-offs attached to the sensors are received by an Analog to Digital (xe2x80x9cA/Dxe2x80x9d) converter. The converted digital signals are applied to a standard digital processor. The digital processor is a processing unit that executes machine readable instructions that are stored in a memory connected to the processor via a bus. A typical digital processor has a Read Only Memory (ROM) which stores the instructions for performing desired processes such as the processes of the present invention. The processor is also connected to a Random Access Memory which stores the instructions for a process that is currently being executed and the data needed to perform the process. The processor may also generate drive signals for the Coriolis flowmeter. In order to apply the drive signal to a drive system, a digital processor may be connected to a Digital to Analog (D/A) convertor which receives digital signals from the processor and applies analog signals to the drive system.
The processes of the present invention perform the following functions to determine the frequencies of the signals received from the pick-off sensors as well as the phase difference between the signals. First, the signals are received from the pick off sensor at a first sample rate. A sample rate is the amount of inputs received from the pick-offs that are used to characterize the signals from the pick-offs. The signals are then decimated from a first sample rate to a desired sample rate. Decimation is simply converting from a first number of samples to a lesser number of samples. Decimation is performed to increase the resolution of the signals sampled to provide a more precise calculation of signal frequency for each signal. The frequency of each signal is then determined.
In order to use the same processes with different flowmeters having different frequencies, the following steps may also be performed. An estimate of the oscillation frequency of the flowmeter is calculated. The estimated frequency is then used to demodulate the signals from each pick-off into an I component and a Q component. The I component and the Q component are then used to translate the signals to a center frequency if the operating frequency of the signals is greater than a transition frequency. After demodulation, the signals may be decimated a second time to improve the resolution of the signals a second time.
The dominant frequency of the signals is then isolated and precisely measured. The translation to a zero frequency is then calculated for both the I component and Q components of the signals. At this time, each component may decimated again to improve the accuracy of measurement. The frequency band of each signal can be narrowed as much as desired by appropriate low pass filtering at this time. A complex correlation is then performed which determines the phase difference between the signals.
The above process allows a low power, low cost, digital processor to be used in a different types of Coriolis flowmeter which operate over a wide range of operating frequencies.